Craft Practice Questions

m.j.w

Hi, have a few questions from craft practice that im having trouble with.

-What is the resistance of 50 metres of cable if 100 metres of the cable is 2MOhms

Is this question saying that there is a fault in the cable as the resistance is 2MOhms? And if you measure the resistance of 50m who is to say the fault isnt still in the cable. Is this not a trick question or is the answer 1MOhm?

-What do you set the overload to on a star delta starter?

Seen this question on a few of the papers and there is nothing on it in the notes.

bassey

Not a trick question, just a stupid one. If 100 metres is 2 Mega Ohms then 50 metres is 1 Megohm.

For a Star/Delta starter the overload should be set at 57.7% of the rated full load current of the motor. This is because the overload units are set to monitor the phase current rather than the line current.

m.j.w

thanks, yea that is a stupid question and thats from a past paper

ZENER

bassey wrote: »

Not a trick question, just a stupid one. If 100 metres is 2 Mega Ohms then 50 metres is 1 Megohm.

Are you sure ? I presume as the resistance is given in Megohms then you are measuring the resistance of the insulation. In which case the resistance of 50 meters would be greater. If you take each meter of insulation as having a given resistance then 50 meters would be all that resistance in parallel - so if 50 meters was 2Mohm then 100 would be 100 times the resistances in parallel which would be less not more than the 50m resistance wouldn't it ?

Or maybe the OP made a typo and the readings were ohms rather than megohms in which case the 100mtr cable would have twice the resistance of the 50 mtr one as the resistance of the conductor is being measured.

Ken

bassey

I was just reading it as it was given, surely it would say if it meant insulation resistance?

ZENER

A reasonable assumption but given the resistances could you see any use in a cable with a 2MOhm resistance ? I'd be inclined to read it as the Megger reading. Unless of course the OP did a typo !!

Ken

2011

ZENER wrote: »

A reasonable assumption but given the resistances could you see any use in a cable with a 2MOhm resistance ? I'd be inclined to read it as the Megger reading. Unless of course the OP did a typo !!

Ken

I'm with bassey on this one.

I agree that 2M ohms is very high for cable resistance, but it is also very low for insulation resistance.

Normally the insulation resistance for a 100m of a cable you would meggar would read off the scale.

I feel that you just have to take this question as it is given.

Either way it is a stupid question.

M cebee

odd one

if it was IR you couldn't really predict what the IR of 50m is



also in theory there's 3 variations on overload 'siting' for star-delta

(windings,delta leg and supply lines iirc)
and 1 of 'em is 1*FLA and others are 0.58*FLA

m.j.w

nope not a typo, thats the right figure and thats the way its worded. hopefully it wont come up in exam

.G.

what phase are you in?

m.j.w

phase 4 why ya ask?

Bruthal

2011 wrote: »

I'm with bassey on this one.

I agree that 2M ohms is very high for cable resistance, but it is also very low for insulation resistance.

Normally the insulation resistance for a 100m of a cable you would meggar would read off the scale.

I feel that you just have to take this question as it is given.

Either way it is a stupid question.

Yes seems a ridiculous stupid question without a doubt, its almost as if its meant to be milli-ohms or something, which would be very low for large cables. 3 or 4 milliohms for 50 meters of 240 square cable i think.

M cebee wrote: »

odd one

if it was IR you couldn't really predict what the IR of 50m is

Yea id fully agree with that, you would not be trying to find insulation resistances for cable lengths. Just insulation resistance for circuits etc.

Thats why bassey and 2011 are right to take it as asking what the conductor resistance is when the cable length is doubled.

also in theory there's 3 variations on overload 'siting' for star-delta

(windings,delta leg and supply lines iirc)
and 1 of 'em is 1*FLA and others are 0.58*FLA

The standard would be the delta leg usually i think? So 0.58 x FLA alright.

.G.

m.j.w wrote: »

phase 4 why ya ask?

questions as stupid as that often came up in phase 2 and the answer was the obvious one that you figured it was.It was asked to see if you knew what happened to resistance of a conductor when its length was changed as opposed to its CSA.

I would assume its the same reasoning in phase 4 but I never came across it myself during that phase.

Its not unknown for incredibly stupid questions to be asked in these exams in all the phase's,noticed some myself while revising for my phase 6 exams a few months back.

we were always told that if a question came up that was open to interpretation then bring it to the attention of those in charge at the exam and let them make a decision on it. we were told not to answer a particular question in craft practice for phase 6 cos it was badly worded.

m.j.w

just one more question lads, test is on monday!

How does the application of standby lighting differ from that of emergency lighting?

Cant answer this one, is standby lighting not a type of emergency lighting?

Bruthal

m.j.w wrote: »

just one more question lads, test is on monday!

How does the application of standby lighting differ from that of emergency lighting?

Cant answer this one, is standby lighting not a type of emergency lighting?

I would say emergency lighting provides enough lighting to find your way out of a building during normal lighting failure, where as standby lighting provides enough light to enable normal activities to continue in the building in the event of normal lighting failure.

Others might correct me there though.

bassey

I'd agree with Robbie. Emergency lighting is just the bare minimum to get out of the building safely. Standby lighting would be the full installation being lit.

Best of luck on Monday, I've my craft practice exam on Tuesday.

m.j.w

your in phase 6 arent ya?

bassey

Nah I'm in phase 4

m.j.w

ah right ill try and remember some of the questions for ya. ill pm you tomorrow

crasy dash

@ m.j.w do you have phase 6 done?

m.j.w

no im doing phase 4 tests tomorrow

crasy dash

ah alrite lad best of luck what college you in

m.j.w

dundalk. u in phase 6?

crasy dash

ah yeah was just looking for someone with notes that were any good

Sound anyway

m.j.w

had test today went well. just one question. can storage heater be on the same night saver meter as the other general circuits? i wrote yes using a dual tariff meter. is that right?

Marcus_Crassus

That's correct. Night Storage heaters can only be used at night due to the ESB's timeclock so they have a restricted supply. But yes, you can use anything at the nightsaver rate.

M cebee

yes for dual-tarriff all off-peak usage is at the cheaper rate

but for separate NSH meter only storage heating has the cheaper rate

Bruthal

I think m.j.w`s question was, can the storage heaters use the same actual meter in a dual tarrif setup, as the rest of the installation, as in a single meter for everything.

M cebee

yes anyhow